The A500 (and for that matter, A600 and 1200 too) have a separate line for the Ctrl+A+A signal, and all "big box" Amigas have the same scheme for the reset as the A1k.
So grab any of those BB keyboards, make the adaptor and you are done.
BTW: all A500 uses the same special reboot line, even those older ones (dunno about the very 1st version, as it is basically an A1000 in other case.
